Ultraviolet (UV) light affects the human body in both beneficial (e.g., vitamin D and tanning) and detrimental (e.g., skin wrinkling, skin cancer, and sun burn or erythema) ways. UV light is typically more difficult to measure than visible and near infrared light because the spectral content is much weaker than visible light and the short wavelength provides an abundance of challenges for detection systems. In both UV and visible light systems optical filters are typically angle sensitive, where the passband shifts to shorter wavelengths at higher angles of incidence, limiting the useful field of view of a sensor. Solutions are desirable for converting a measured spectrum to a desired spectrum by compensating for the difference between a measured and target spectrum as well as the difference in desired field of view.
The UV spectrum is made up of three regions: UVA, UVB and UVC. Solar UVC radiation is blocked by the earth's atmosphere. Solar UVB light is partially blocked by the stratospheric ozone layer, and UVA light largely transmits. Both UVA and UVB light experience significant Rayleigh scattering, the phenomenon responsible for making the sky blue. The UVB spectral range (˜280-315 nm) includes shorter wavelengths than the UVA spectral range (˜315-400 nm) and is mostly responsible for sunburn, carcinoma of the skin and vitamin D generation. UVA includes longer wavelengths that cause tanning, freckles and skin aging effects.
The shorter wavelengths of UV light pose challenges for efficient detection with common photodiode materials. To detect UV light, either a special shallow junction photodiode in a typical Optoelectronic material such as silicon can be used, such as a lateral junction on SOI or a lower volume supply, wide bandgap material (e.g., SiC or AlGaN). In this context, measuring UVB is much more challenging than measuring UVA. Most optical window or lens materials are highly or partially transmissive to UVA. Few are highly transmissive to UVB, and they are usually more expensive. Additionally UVA is 20% bandwidth and UVB is 10% bandwidth, which makes optical filter design more challenging for UVB and more susceptible to angle-dependency. Additionally, the filter layers are thinner and thickness control tolerances rapidly become critical to a costly degree. Lastly and most importantly, there is very little UVB in the solar spectrum at any given time, approximately 0-4% of the total UV radiation depending on the atmospheric conditions. For example, FIG. 5 shows a spectral photocurrent response of a lateral junction photodiode with visible blocking filter overlaid with a solar spectrum. As can be seen in FIG. 5, the UVB spectral contributions to the detected light are much lower than the UVA spectral contributions.
UVB is not only hard to measure with a detector; it is also challenging to manufacture a UVB detector in a cost-effective manner due to the tight tolerances needed for filter response, dopant profile, field of view, surface states, strain, and the like. Poor responsivity for narrower bandgap detectors like silicon is compounded by higher dark current. In addition, manufacturing issues abound for wide bandgap semiconductors, particularly dislocation density and yield. In both cases the requisite large area and preference for a diffuser to limit angle-sensitivity of the optical filter magnify the detector sizes and system cost. Trimming and/or calibrating a UVB detector also requires a UVB light source (preferably broadband), such as a Xenon lamp. These light sources tend to be large, bulky costly, noisy, and high maintenance.
Finally, (unless heavily diffused) a typical optical detector has a field of view (FOV) limited by the optical package. UV Index is defined for an ideal planar detector (e.g., having at least 120 degree FOV). A method is needed to relate narrow FOV (e.g., ≦90 degrees) UVA or total UV measurements to what would be measured by a wide FOV UVB or erythema action spectrum weighted detector. A high accuracy solution for estimation of biologically relevant spectral contributions (e.g., human-health relevant UV Index or CIE 1931 XYZ color values) is also desirable with a manufacturable detection system for mobile consumer applications, among others.